This invention relates to testing arrangements for communication switching systems in general and to an arrangement for testing a communication path within a time-division multiplexed (TDM) telephone switching system in particular.
In prior telephone switching systems having analog switching networks, each path through the switching system is tested for continuity prior to establishing a voice path connection. Typically, such continuity tests were provided by d.c. current detectors which would automatically be connected to a path to detect for a d.c. current flow.
One problem unique to digital switching systems is that each voice path is converted into two unidirectional paths, and the aforementioned techniques cannot be applied. In certain digital switching systems having a time-division multiplexed network, the equivalent of the analog continuity test is provided by including additional logic circuits for selectively connecting the transmit and receive paths of a digital line circuit together so that a digital test signal may be transmitted over one path and "looped-back" over the other path to a test circuit. However, such prior arrangements applied to digital systems will not determine if the analog-to-digital and digital-to-analog converters in a line circuit are operational. Such converters are typically complex and may contain relatively large numbers of components and because a large number of such converters are employed in a digital telephone switching system, it is desirable that the paths through the converters be tested.